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I recently started with Java and want to understand a java module of a large app. I came across this line of java code: 

String line = (new BufferedReader(new InputStreamReader(System.in))).readLine();

What does this java code do. Is there a C/C++ equivalent of this?

Shan
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    Why is this tagged C++ and C? – Mats Petersson Feb 06 '13 at 13:36
  • Wrapper and wrapper. Read the javadoc of [`InputStreamReader`](http://docs.oracle.com/javase/7/docs/api/java/io/InputStreamReader.html) and [`BufferedReader`](http://docs.oracle.com/javase/7/docs/api/java/io/BufferedReader.html) first. – Alvin Wong Feb 06 '13 at 13:37
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    C++ equivalent would roughly be `std::string line; std::getline(std::cin, line);` – jrok Feb 06 '13 at 13:38

5 Answers5

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System.in is the standard input.

InputStreamReader allows you to associate a stream that reads from the specified input (in this case the standard input), so now we have a stream.

BufferedReader is an "abstraction" to help you to work with streams. For example, it implements readLine instead of reading character by character until you find a '\n' to get the whole line. It just returns a String after this proccess.

So this line means: "Read a line from standard input and store it in line variable".

arutaku
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  • /facepalm. `System.in` is the `InputStream` of the standard input. `InputStreamReader` [_is a bridge from byte streams to character streams_](https://docs.oracle.com/javase/8/docs/api/java/io/InputStreamReader.html). `BufferedReader` is a character stream (`Reader`) for [efficient reading of characters, arrays, and lines](https://docs.oracle.com/javase/8/docs/api/java/io/BufferedReader.html), not _an "abstraction" to help you to work with streams_. – bit2shift Jul 20 '18 at 17:02
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> What does this java code do:

String line is your string object

new BufferedReader().readLine() is the instance of a BufferedReader to read text from a character input stream; and readline() is a method it implements to read until a newline character.

new InputStreamReader() gives you a instance of an InputStreamReader which is the "bridge" between the standard in byte stream and the character stream which a BufferedReader wants.

System.in is the standard input (byte stream)

> Is there a C/C++ equivalent of this
Well... there's no language called C/C++... ;)
So I'll assume you wanted an answer for each of them.

In C, there are no "strings" you have to use a character array, but you can read data in to a character array from stdin with something like:

char input[100];
...
scanf("%99[^\n]", input);

or

fgets (input, 100 , stdin)

In C++, you'd use:

using namespace std;

string line;
getline(cin, line);
Mike
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Your snippet uses a BufferedReader, chained to an InputStreamReader, to read aline from the standard input console and store it to the String line .

BufferedReader

Read text from a character-input stream, buffering characters so as to provide for the efficient reading of characters, arrays, and lines. The buffer size may be specified, or the default size may be used. The default is large enough for most purposes. In general, each read request made of a Reader causes a corresponding read request to be made of the underlying character or byte stream. It is therefore advisable to wrap a BufferedReader around any Reader whose read() operations may be costly, such as FileReaders and InputStreamReaders.

BufferedReader#readLine()

Read a line of text. A line is considered to be terminated by any one of a line feed ('\n'), a carriage return ('\r'), or a carriage return followed immediately by a linefeed.

InputStreamReader

An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted. Each invocation of one of an InputStreamReader's read() methods may cause one or more bytes to be read from the underlying byte-input stream. To enable the efficient conversion of bytes to characters, more bytes may be read ahead from the underlying stream than are necessary to satisfy the current read operation.

System

The System class contains several useful class fields and methods. It cannot be instantiated.

Among the facilities provided by the System class are standard input, standard output, and error output streams; access to externally defined "properties"; a means of loading files and libraries; and a utility method for quickly copying a portion of an array.

System.in

The "standard" input stream. This stream is already open and ready to supply input data. Typically this stream corresponds to keyboard input or another input source specified by the host environment or user.

Achintya Jha
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1

What the code does is just simply read a line from input stream. from pattern point of view, this is a decorator. As to using BufferedReader is aiming to improve IO performance.

spiritwalker
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    Using BufferedReader is for the `readLine()` method, which doesn't exist in InputstreamReader. – davmac Feb 06 '13 at 13:41
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An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.

Each invocation of one of an InputStreamReader's read() methods may cause one or more bytes to be read from the underlying byte-input stream. To enable the efficient conversion of bytes to characters, more bytes may be read ahead from the underlying stream than are necessary to satisfy the current read operation.

For top efficiency, we consider wrapping an InputStreamReader within a BufferedReader. For example:

BufferedReader in = new BufferedReader(new InputStreamReader(System.in));

PeteBeat
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